Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. In particular, an increased volumetric capacity density has been desired. Because of their high energy density and high capacity, non-aqueous electrolyte secondary batteries, such as represented by lithium secondary batteries, are widely utilized as the driving power sources for such mobile information terminals.
A non-aqueous electrolyte secondary battery as described above generally employs a positive electrode containing a positive electrode active material made of a lithium-containing transition metal composite oxide, a negative electrode containing a negative electrode active material made of a carbon material capable of intercalating and deintercalating lithium, such as graphite, and a non-aqueous electrolyte in which an electrolyte made of a lithium salt, such as LiBF4 and LiPF6, is dissolved in an organic solvent such as ethylene carbonate and diethyl carbonate. In this kind of battery, the charge-discharge operations are performed by migration of lithium ions between the positive and negative electrodes.
In recent years, as the number of functions of mobile information terminals has increased, the power consumption of the devices has been increasing. Accordingly, demand has been escalating for non-aqueous electrolyte secondary batteries that achieve further higher energy density.
In order to achieve a higher energy non-aqueous electrolyte secondary battery, it is necessary to use not only a high energy density negative electrode active material but also a high energy density positive electrode active material. In view of such circumstance, lithium-containing layered compounds such as LiCoO2, LiNiO2, and LiNi1/3Co1/3Mn1/3O2 have been proposed. In addition, sodium oxide has also drawn attention because many types of layered compounds that are difficult to synthesize with the use of lithium oxide are easily synthesized with sodium oxide. In particular, Na0.7CoO2 and NaCo1/2Mn1/2O2 can be used as a positive electrode active material of the non-aqueous electrolyte battery by ion-exchanging sodium for lithium. Therefore, much research has been conducted on ion-exchange methods using a synthesis technique and a chemical technique using such substances (see Patent Documents 1 and 2 and Non-patent Documents 1 and 2 mentioned below).
[Patent Document 1] Japanese Published Unexamined Patent Application No. 2002-220231
[Patent Document 2] Japanese Published Unexamined Patent Application No. 2001-328818
[Non-patent Document 1] Akihisa. Kajiyama et al., Solid State Ionics, 149 (2002), 39-45
[Non-patent document 2] F. Tournadre et al, J. Solid State Chem. 177 (2004), 2790-2802